1. Field of the Invention
The invention relates to a halftone dot image recording apparatus wherein density of a screen pattern signal is increased thereby to substantially enchance reproducibility of gradation.
2. Description of the Prior Art
In a halftone dot image recording apparatus such as a process scanner of electronic control type, a level of an image signal is compared with a level of a prescribed screen pattern signal, and a photosensitive material is scanned by an exposure of light beam which is turned on and off according to a result of such comparison, to record a halftone dot image thereon. The screen pattern signal is generated based on the screen pattern data which are prepared previously.
As well known in the art, the screen pattern data express a threshold level distribution in the halftone dot in which a small area A.sub.ij shown in FIG. 1 (hereinafter called as "unit area") is employed as a quantized space unit, and the unit area A.sub.ij is obtained by dividing one halftone dot HD into lattice or matrix. FIG. 2 shows a state in which the screen pattern data prepared for the halftone dot HD of FIG. 1 is stored in a memory and numerals in FIG. 2 represent data value given to each unit area A.sub.ij. A shaded unit area shown in FIG. 1 represents an area which has a screen pattern data that is greater than "128" and a region on the photosensitive material corresponding to the shaded area is exposed by the light beam in recording a halftone dot with respect to an image signal having a level of "128".
The size of the unit area A.sub.ij is determined according to light spot diameter d of the light beam to expose at a position on an image recording surface of the photosensitive material. More specifically, the size of the unit area A.sub.ij is determined so that the light spot diamter d will be a range of 0.75 to 1.5 times a length a of one side of the unit area A.sub.ij (the length of the side is hereafter called as "unit area size"). In FIG. 1, a diameter of a circle which inscribes the unit area A.sub.ij is employed as the light spot diameter d for convenience of illustration.
Turning on and off of the light beam for exposure is judged every time a scanning is advanced by a distance according to the light spot diameter d, and the intensity of the light beam is subjected to turning on and off according to the results of the judgement. Therefore, in a conventional halftone dot image recording apparatus, a space interval capable of the beam intensity change is restricted to the integral multiples of the unit area size a.
As a result, in a square halftone dot having a screen pitch K where the following relation (1) holds, EQU K=Na (N is an intergral number) (1)
gradation number of recorded image has a maximum value M of: EQU M=(K/a).sup.2 =N.sup.2 ( 2)
Therefore, in order to increase the gradation number M, it is necessary to increase the value (K/a) in the formular (2). In a flat bed type scanner, however, since it is necessary to secure a certain degree of scanning length, long focal distance of an image forming lens of a recording optical system must be provided. Therefore, in the flat bed type scanner, the light spot diameter of the light beam for exposure at a position of the photosensitive material cannot be made too small, and to realize the light spot diameter of about 15 to 16 .mu.m for example involves extremely technical difficulties and increased costs. Consequentially, in the flat bed type scanner, the unit area size a is also relatively large and there exists a condition where the gradation number M is compelled to become small according to the formula (2).
In a drum type scanner which is capable of making the light spot diameter d and the unit area size a to be relatively small as well, when the screen pitch K is small, the gradation number M also becomes low according to the formula (2).
As stated above, in the conventional halftone dot image recording apparatus, there is a problem wherein the gradation number is low when the ratio of the screen pitch K to the unit area size a cannot be made large, and the gradation reproducibility of the recorded becomes reduced.
Furthermore, because there is a necessity to secure the spatial periodicity of the halftone dot in the image recording, the screen pitch K depends upon the size of the light spot diameter d (therefore the unit area size being a). In an example of the halftone dot having a screen angle of 0.degree. as shown in FIG. 1, the screen pitch K is restricted by the integer multiple of the unit area size a. Therefore, in a conventional apparatus, there is a problem of low degree of freedom in selection of the screen pitch.